System and method for tracking the location of multiple mobile radio transceiver units

ABSTRACT

An object locating system ( 100 ), the system ( 100 ) comprising at least first and second mobile devices ( 120, 130 ) associated by a relationship defined at least in part by a threshold. The at least first and second mobile devices are operable to be monitored by a communication network as one mobile device ( 120 ) or as separate mobile devices ( 120, 130 ) as a function of the relationship relative to the threshold.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to location trackingsystems and, more specifically, to a system for tracking the locationsof multiple mobile units that communicate with a central monitoringdevice by a radio frequency (RF) link.

BACKGROUND OF THE INVENTION

Tracking the location or position of a body that can move in unknowndirections over a considerable range of territory has been a concern fora number of years. The term “body” is defined broadly, meaning anyorganic or inorganic object whose movement or position may suitably beevaluated relative to its environment in accordance with the principleshereof. The term “environment” is defined broadly as conditions andinfluences that define, at least in part, the physical system in whichthe body is located.

In the past, locating systems were proposed that employed conventionalwireless technology, but which tended to be cumbersome, bulky,expensive, and lack robustness, or all of the above. With the deploymentof the global positioning system (GPS), it is now possible to providerelatively inexpensive location systems for determining body location.Such systems have been utilized, for example, on trucks to providelocation information for companies having large fleets of trucks in useat any one particular time. The position of each individual truck maytherefore be tracked.

Conventional GPS devices have disadvantages, including relative slownessin acquiring location data, being strongly dependent upon the targetbody being in an open area where it is in a line-of-sight position tothe GPS satellites, and consuming a relatively large amount ofelectrical power during the period in which the location information isbeing acquired from the GPS satellites.

The inventor hereof disclosed various methodologies for an improvedobject locator system in the following U.S. patents:

1) No. 6,518,919, entitled “Mobile Object Locator”;

2) No. 6,480,147, entitled. “Portable Position Determining Device”;

3) No. 6,441,778, entitled “Pet Locator”;

4) No. 6,421,001, entitled “Object Locator”;

5) No. 6,236,358, entitled “Mobile Object Locator” and

6) No. 6,172,640, entitled “Pet Locator”.

These patents disclose an object locator system that requests andobtains information about the location of a mobile object, havingattached thereon a lightweight object locator that is operable in aregion served by a two-way paging system and a global positioningsystem. The object locator may be activated selectively to conservepower or enabled to respond only when beyond or within a definedboundary. These patents also teach that the object locator systemprovides the location information in several forms.

However, the methodologies disclosed in these patents fail to includethe robustness necessary to track groups of bodies/objects that arerelated, permanently or temporarily, within one or more environments.Further, these methodologies fail to understand the complexrelationships between related bodies/objects, whether such relationshipsare hierarchical or otherwise. Further yet, these methodologies fail toadequately address continued (though reduced) line-of-sight and powermanagement issues.

Therefore, there is a need in the art for an improved system forgathering the location information of one or more tracked objects.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, it is aprimary object of the present invention to provide a object locatingsystem, the system comprising at least first and second mobile devicesassociated by a relationship defined at least in part by a threshold,wherein the at least first and second mobile devices are operable to bemonitored by a communication network as one mobile device or as separatemobile devices as a function of the relationship relative to thethreshold.

In one embodiment of the present invention, the threshold is based upondistance between the at least first and second mobile devices.

In another embodiment of the present invention, the relationship betweenthe at least first and second mobile devices is hierarchical.

In still another embodiment of the present invention, the hierarchicalrelationship is master-slave between the at least first and secondmobile devices.

In yet another embodiment of the present invention, the relationship isdynamic.

In a further embodiment of the present invention, the relationship isstatic.

In a still further embodiment of the present invention, thecommunications network uses passive tags to monitor mobile devices.

In one embodiment of the present invention, the communications networkis capable of tracking monitored mobile devices relative to anenvironment, such as a position within a geographical boundary.

Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, itmay be advantageous to set forth definitions of certain words andphrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; and theterm “controller” means any device, system or part thereof that controlsat least one operation, such a device may be implemented in hardware,firmware or software, or some combination of at least two of the same.It should be noted that the functionality associated with any particularcontroller may be centralized or distributed, whether locally orremotely. Definitions for certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 illustrates a location tracking system according to anadvantageous embodiment of the present invention;

FIG. 2 illustrates a primary base station in greater detail according toan exemplary embodiment of the present invention;

FIG. 3 illustrates an alternate method of determining the location of amobile unit according to one embodiment of the present invention; and

FIG. 4 illustrates another alternate method of determining the locationof a mobile unit according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 4, discussed below, and the various embodiments used todescribe the principles of the present invention in this patent documentare by way of illustration only and should not be construed in any wayto limit the scope of the invention. Those skilled in the art willunderstand that the principles of the present invention may beimplemented in any suitably arranged wireless communication network.

FIG. 1 illustrates location tracking system 100 according to anadvantageous embodiment of the present invention. Location trackingsystem 100 comprises primary base station 110, primary mobile unit 120,and up to “N” secondary mobile units, including exemplary secondarymobile units 130 and 140, associated with primary mobile unit 120. Eachone of primary mobile unit 120 and the N secondary mobile units isattached to a person, animal, vehicle or other mobile object that isbeing tracked. Location tracking system 100 may comprise additionalprimary base stations similar to primary base station 110 and additionalprimary mobile units similar to primary mobile unit 120. However, toavoid redundancy and to simplify the explanation of the presentinvention, only primary base station 110 and primary mobile unit 120 areshown.

Location tracking system 100 is an improvement on the mobile objectlocating system and related components thereof that are disclosed innumerous U.S. patents to Durst et al., including U.S. Pat. No.6,172,640, U.S. Pat. No. 6,236,358, U.S. Pat. No. 6,421,001, U.S. Pat.No. 6,441,778, U.S. Pat. No. 6,480,147 and U.S. Pat. No. 6,518,919. Thesubject matter disclosed in each of U.S. Pat. No. 6,172,640, U.S. Pat.No. 6,236,358, U.S. Pat. No. 6,421,001, U.S. Pat. No. 6,441,778, U.S.Pat. No. 6,480,147 and U.S. Pat. No. 6,518,919 (collectively referred tohereafter as “the Durst patents”) is hereby incorporated into thepresent disclosure as if fully set forth herein.

Primary base station 110 comprises antennas 111, 112 and 113. Accordingto an exemplary embodiment of the present invention, primary basestation 110 comprises a Global Positioning System (GPS) receiver thatdetermines the position of primary base station 110 using radiofrequency signals received from global positioning satellite system 190via antenna 111. Primary base station 110 also comprises a 2-way pagingtransceiver that communicates with one or more of primary mobile unit120 and the N secondary mobile units (e.g., units 130 and 140). Thepaging transceiver communicates bi-directionally with 2-way pagingsystem 180 via antenna 112. Finally, primary base station 110 comprisesa local RF transceiver that communicates directly with one or more ofprimary mobile unit 120 and the N secondary mobile units over relativelyshort distances using antenna 113.

Additionally, primary base station 110 is capable of communicating overline 115 via a wireline interface with an external communication network(not shown), such as the Internet or an Ethernet local area network(LAN). This allows primary base station 110 to exchange locationinformation with other primary base stations.

Primary mobile unit 120 comprises antennas 121, 122 and 123. Accordingto an exemplary embodiment of the present invention, primary mobile unit120 comprises a Global Positioning System (GPS) receiver that determinesthe position of primary mobile unit 120 using radio frequency signalsreceived from global positioning satellite system 190 via antenna 121.Primary mobile unit 120 also comprises a 2-way paging transceiver thatcommunicates with primary base station 110 and the N secondary mobileunits (e.g., units 130 and 140). The paging transceiver communicatesbi-directionally with 2-way paging system 180 via antenna 122. Finally,primary mobile unit 120 comprises a local RF transceiver thatcommunicates directly with one or more of primary base station 110 andthe N secondary mobile units over relatively short distances usingantenna 123.

Secondary mobile unit 130 comprises antennas 131, 132 and 133. Accordingto an exemplary embodiment of the present invention, secondary mobileunit 130 comprises a Global Positioning System (GPS) receiver thatdetermines the position of secondary mobile unit 130 using radiofrequency signals received from global positioning satellite system 190via antenna 131. Secondary mobile unit 130 also comprises a 2-way pagingtransceiver that communicates with primary base station 110, primarymobile unit 120, and the remaining N-1 secondary mobile units (e.g.,unit 140). The paging transceiver communicates bi-directionally with2-way paging system 180 via antenna 132. Finally, secondary mobile unit130 comprises a local RF transceiver that communicates directly with oneor more of primary base station 110, primary mobile unit 120, and theremaining N secondary mobile units over relatively short distances usingantenna 133.

Secondary mobile unit 140 is identical in nearly all respect tosecondary mobile unit 130. Secondary mobile unit 140 comprises antennas141, 142 and 143. Secondary mobile unit 140 comprises a GlobalPositioning System (GPS) receiver that determines the position ofsecondary mobile unit 140 using radio frequency signals received fromglobal positioning satellite system 190 via antenna 141. Secondarymobile unit 140 also comprises a 2-way paging transceiver thatcommunicates with primary base station 110, primary mobile unit 120, andthe remaining N-1 secondary mobile units (e.g., unit 130). The pagingtransceiver communicates bi-directionally with 2-way paging system 180via antenna 142. Finally, secondary mobile unit 140 comprises a local RFtransceiver that communicates directly with one or more of primary basestation 110, primary mobile unit 120, and the remaining N secondarymobile units over relatively short distances using antenna 143.

FIG. 2 illustrates primary base station 110 in greater detail accordingto an exemplary embodiment of the present invention. It should be notedthat primary mobile unit 120 and secondary mobile units 130 and 140 areidentical in most respects to primary base station 111. Therefore, thefollowing description of primary base station 111 also applies toprimary mobile unit 120 and secondary mobile units 130 and 140, exceptas noted otherwise.

Primary base station 110 comprises GPS receiver 210, paging transceiver220 and local radio frequency (RF) transceiver 230. GPS receiver 210,paging transceiver 220 and local RF transceiver 230 communicate viaantennas 111, 112 and 113, respectively, as described above in FIG. 1.Primary base station 110 also comprises controller 240, input/output(I/O) interface 250, user input device 260 (e.g., keyboard), and display270 (e.g., digital alphanumeric readout, LCD display, flat panel videomonitor, or the like).

Controller 240 controls the overall operation of primary base station110, including the operation of GPS receiver 210, paging transceiver 220and local RF transceiver 230. According to an exemplary embodiment,controller 240 may comprise a data processor and an associated memory,wherein the data processor executes an operating system program storedin the memory. Controller 240 uses I/O interface 250 to communicate withexternal systems across wireline 115. These external systems may includeother primary base stations. Controller 240 also receives user commandsand data via user input device 260 and outputs data, alerts,instructions and the like to the user via display 270.

Primary base station 110 monitors the locations of one of more primarymobile units and one or more secondary mobile units that are attachedto, or carried by, for example, a group of persons, a group of animals,a group of vehicles, and the like. According to the principles of thepresent invention, primary base station 110 may communicate directlyonly with primary mobile units, using either paging transceiver 220 orlocal RF transceiver 230. In this scenario, primary base station 110communicates with secondary mobile units indirectly using primary mobileunit 120. In this manner, location information is relayed from secondarymobile units 130 and 140 to primary base station 110 through primarymobile unit 120. According to the principles of the present invention,secondary mobile stations 130 and 140 may communicate with each otherand with primary mobile unit 120 using either paging transceiver 220 orlocal RF transceiver 230. Preferably, local RF transceiver 230 is usedto relay location information, thereby limiting the usage of 2-waypaging system 180.

Advantageously, relaying position information through primary mobileunit 120 limits the number of mobile units with which primary basestation 110 must communicate directly. This reduces the overheadmessaging and bandwidth usage through 2-way paging system 180 and/orthrough local RF transceiver 230.

It is not required that primary base station 110, primary mobile unit120, and secondary mobile units 130 and 140 contain three separatereceiver or transceivers. Some state of the art RF transceivers arecapable of operating (sometimes simultaneously) in a wide range offrequency bands using a number of different modulation techniques andprotocols. Thus, for example, paging transceiver 220 and local RFtransceiver 230 may actually be a single transceiver that communicatesdirectly (i.e., transceiver to transceiver) with other mobile units orprimary base stations or communicates with other mobile units or primarybase stations via 2-way paging system 180.

According to an advantageous embodiment of the present invention, localRF transceiver 230 is a relatively short-range device that communicatesover ranges of, for example, 500 yards or less. For example, local RFtransceiver 230 may be an 802.11 transceiver that communicates withanother local RF transceiver 230 according to a wireless Ethernetprotocol. Alternatively, local RF transceiver 230 may communicate overshort distances with another local RF transceiver 230 according to aBluetooth protocol. However, when a mobile unit is out of range ofprimary base station 110 or other mobile units, the mobile unit usespaging transceiver 220 to communicate bi-directionally with primary basestation 110 or other mobile units.

It should be noted that primary base station 110 is not required to bestationary and may, in fact, be a mobile device. The term “primary basestation” is intended to designate primary base station 110 as a centralmonitoring device to which mobile unit location information istransmitted, either directly or by relay. This is true whether primarybase station 110 is located at a fixed geographical position or is onthe move.

Also, it should be noted that a mobile unit may be switched betweenbeing a primary mobile unit or a secondary mobile unit. A mobile unit isa primary mobile unit if it communicates with primary base station 110.A mobile unit is a secondary mobile unit if it communicates only withanother mobile unit (primary or secondary) that, in turn, relayslocation information to primary base station 110.

It may be helpful at this point to provide examples of the operation ofbase stations and mobile units in location tracking system 100. In afirst example, a tour group at a large amusement park uses base stationsand mobile units to monitor members of the tour group. The tour guide isequipped with primary base station 110. A first parent chaperone isequipped with a first primary mobile unit 120A and a second parentchaperone is equipped with a second primary mobile unit 120B. Eachstudent in the tour group is equipped with a secondary mobile unit 130.

If the tour group stays together, the secondary mobile unit used by eachstudent transmits position location information to either first primarymobile unit 120A or second primary mobile unit 120B. For example, themobile units used by all female students may communicate with firstprimary mobile unit 120A used by the first parent chaperone and themobile units used by all male students may communicate with secondprimary mobile unit 120B used by the second parent chaperone. The mobileunits may be assigned in this manner by manual inputs on user inputdevice 260. Since all group members are in close proximity, thecommunications between the primary mobile units and secondary mobileunits generally will be conducted by local RF transceiver 230.

The student position information is relayed by first primary mobile unit120A and second primary mobile unit 120B to primary base station 110.Since all group members are in close proximity, the communicationsbetween the primary mobile units and primary base station 110 generallywill be conducted by local RF transceiver 230. However, in someembodiments, paging transceiver 220 may be used by default. If thestudents and chaperones move away from the tour guide, then pagingtransceiver 220 must be used.

At this point, the second parent chaperone and the male students maybreak away from the first parent chaperone and the female students.Nonetheless, the position information of all male students and thesecond parent chaperone continues to be transmitted by the secondprimary mobile unit 120B to primary base station 110.

According to an advantageous embodiment of the present invention, secondprimary mobile unit 120B used by the second parent chaperone mayoriginally operate as a secondary mobile unit. Thus, the positioninformation of all students and the second chaperone is transmitted tothe first primary mobile unit 120A for subsequent relay to primary basestation 110. In this situation, the secondary mobile unit used by thesecond parent chaperone may or may not receive and relay locationinformation from the secondary mobile units used by the male students.

However, at some point, when the second parent chaperone moves away fromthe first parent chaperone, the secondary mobile unit used by the secondparent chaperone will lose contact with the first primary mobile unit120A used by the first parent chaperone. When this occurs, the secondarymobile unit used by the second parent chaperone will switch fromsecondary mode to primary mode and become primary mobile unit 120B.After the mode switch, primary mobile unit 120B used by the secondparent chaperone will establish a new connection to primary base station110 via 2-way paging system 180 and will begin to relay its own positioninformation and the position information of nearby male students toprimary base station 110.

According to an advantageous embodiment of the present invention, eachsecondary mobile unit (e.g., units 130 or 140) is capable of searchingfor an alternate primary mobile unit 120 if the secondary mobile unitloses contact with its assigned primary mobile unit 120. Thus, forexample, if the second parent chaperone and most of the male studentsmove away from the first parent chaperone, but one male student remainswith the first parent chaperone and the female students, secondarymobile unit 130 used by the separated male student will eventually losecontact with the second primary mobile unit 120B used by the secondparent chaperone. When this happens, secondary mobile unit 130 willsearch for and acquire the first primary mobile unit 120A used by thefirst parent chaperone. At this point, the first primary mobile unit120A will relay the location information of the separated male studentto primary base station 110.

It should also be noted that the components of location tracking system100 are not exclusively dependent on global positioning satellite system190. According to an advantageous embodiment of the present invention,location tracking system 100 is preferably a GPS-assisted system that iscapable of determining the location information of mobile units usingalternative devices, both active and passive. This is particularlyadvantageous because GPS receivers are line-of-sight devices and oftendo not operate indoors.

FIG. 3 illustrates an alternate method of determining the location of amobile unit according to one embodiment of the present invention. In theillustrated embodiment, a mobile unit (e.g., primary mobile unit 120 orsecondary mobile unit 130) obtains location information from transponder310 attached to wall 320 of a building. Transponder 310 may be either anactive or a passive radio frequency (RF) identification (ID) tag,similar to the toll tags commonly used on automobiles. When mobile unit120 (or 130) loses the GPS signals from global positioning satellitesystem 190, local RF transceiver 230 changes mode and automaticallybegins to transmit RF ID signals to transponder 310. If mobile unit 120(130) is sufficiently close to transponder 310, transponder 310transmits back to mobile unit 120 (130) the ID information (i.e.,location information) embedded in transponder 310.

According to an exemplary embodiment of the present invention, the IDinformation in transponder 310 comprises highly precise latitude,longitude, and elevation information, similar to GPS data. Thus, eventhough mobile unit 120 (or 130) is indoors, mobile unit 120 (or 130) canstill determine its location information. The location informationreceived from transponder 310 is then transmitted (directly or by relay)to primary base station 110.

The widespread use of transponders in a building is particularly usefulfor firefighters. A fire chief may dispatch several teams offirefighters into a building. Each team has a team leader who carries aprimary mobile unit 120. The other members of each team carry secondarymobile units 130, 140. As the firefighters move through the building,each mobile unit captures its location information from each transponder310 the mobile unit passes. Secondary mobile units 130 and 140 transmitlocation information to primary mobile unit 120 carried by the teamleader. Primary mobile unit 120 then transmits its location informationand the location information of the secondary mobile units to primarybase station 110. Advantageously, if a team member using secondarymobile unit 130 is separated from the team leader and loses contact withprimary mobile unit 120, secondary mobile unit 130 can switch modes tobecome a primary mobile unit and establish a new communication link toprimary base station 110 via 2-way paging system 180.

FIG. 4 illustrates another alternate method of determining the locationof a mobile unit according to one embodiment of the present invention.According to the alternate embodiment, primary base station 110 may useconventional triangulation techniques to determine the locationinformation of a mobile unit. As FIG. 4 illustrates, primary mobile unit120 is located on an upper floor (labeled 410) inside building 400.Primary base station 110 controls a plurality of antennas, includingantennas 113 a, 113 b and 113 c. Each of antennas 113 a, 113 b and 113 cis positioned at a known geographical location and elevation.

If transponders, such as transponder 310, are not present and GPSsignals are blocked, local RF transceiver 220 in primary mobile unit 120may transmit a beacon signal. The beacon signal is received by each ofantennas 113 a, 113 b and 113 c. Primary base station 110 usesbeamforming or similar techniques on the received beacon signals todetermine the direction in three-dimensional (3D) space of primarymobile unit 120. Conventional triangulation techniques may then be usedto pinpoint the location of primary mobile unit 120.

It should be noted that the foregoing embodiments were introduced forillustrative purposes only and that the present invention broadlyintroduces systems, as well as methods of operating such systems, thatdefine one or more relationships among mobile units monitored by atleast one communications network containing a primary base station. Suchrelationships are defined by an environment, which in the above-givenexample is an tour group environment. An important aspect of the presentinvention is that the controller is operable to process sensed phenomenaas a function of the environment, and that such phenomena will largelybe defined by the specific application. Therefore, the system, and, moreparticularly, the controller, generates state indicia relative to theenvironment of interest, and determines whether the related mobile unitsare monitored as one or two (or more) mobile units in the context ofthat environment and as a function of one or more thresholds. Forinstance, “threshold” would likely be very different for monitoreddevices a group of elderly people on an outing, firemen responding to afire in a high rise, a tour group in an amusement park, a televisioncamera crew reporting at a natural disaster, a tactical military team,or the like. One threshold described above is the loss of signal contactbetween mobile units. In alternated embodiments, other thresholds maytrigger the monitoring of the related mobile units as one or two (ormore) mobile units. For example, if a secondary mobile unit wandersoutside of a defined geographical area, the system may monitor thesecondary mobile unit as a primary unit instead.

Although the present invention has been described with an exemplaryembodiment, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present invention encompasssuch changes and modifications as fall within the scope of the appendedclaims.

1. (canceled)
 2. A tracking system comprising: a first mobile devicecomprising a first position determining apparatus; a second mobiledevice comprising a second position determining apparatus; and a centralmonitoring system for monitoring the positions of the first and secondmobile devices, wherein the first mobile device is capable oftransmitting to the central monitoring system first position informationassociated with the first mobile device and relaying to the centralmonitoring system second position information associated with the secondmobile device.
 3. The tracking system as set forth in claim 2, whereinthe first mobile device relays the second position information to thecentral monitoring system when the second mobile device is within athreshold distance of the first mobile device.
 4. The tracking system asset forth in claim 3, wherein the second mobile device is capable oftransmitting the second position information directly to the centralmonitoring system.
 5. The tracking system as set forth in claim 4,wherein the second mobile device relays the second position informationdirectly to the central monitoring system when the second mobile deviceis not within the threshold distance of the first mobile device.
 6. Thetracking system as set forth in claim 5, wherein at least one of thefirst and second position determining apparatuses comprises a GlobalPositioning System (GPS) unit.
 7. The tracking system as set forth inclaim 5, wherein at least one of the first and second positiondetermining apparatuses comprises a transceiver capable of receivingsignals from an RF transponder.
 8. The tracking system as set forth inclaim 5, wherein a value of the threshold distance is static.
 9. Thetracking system as set forth in claim 5, wherein a value of thethreshold distance is dynamic.
 10. The tracking system as set forth inclaim 5, wherein the central monitoring system monitors the positions ofat least one of the first and second mobile devices while the at leastone of the first and second mobile devices is within a definedgeographical boundary.
 11. The tracking system as set forth in claim 5,wherein the central monitoring system comprises at least a first basestation and a second base station capable of receiving a beacon signalfrom the first mobile device.
 12. The tracking system as set forth inclaim 11, wherein the central monitoring system is capable ofdetermining a position of the first mobile device using a triangulationalgorithm that uses beacon signals received from the first mobile deviceby the first base station and the second base station.
 13. A firstmobile device capable of operating within a tracking system, the firstmobile device comprising: a position determining apparatus fordetermining a position of the first mobile device; and at least onetransceiver capable of communicating with at least a second mobiledevice and with a central monitoring system associated with the trackingsystem, wherein the first mobile device is capable of transmitting tothe central monitoring system first position information associated withthe first mobile device and relaying to the central monitoring systemsecond position information associated with the second mobile device.14. The first mobile device as set forth in claim 13, wherein the firstmobile device relays the second position information to the centralmonitoring system when the second mobile device is within a thresholddistance of the first mobile device.
 15. The first mobile device as setforth in claim 14, wherein the position determining apparatus comprisesa Global Positioning System (GPS) unit.
 16. The first mobile device asset forth in claim 15, wherein the first mobile device transmits alocation determined by the GPS unit to the central monitoring system.17. The first mobile device as set forth in claim 14, wherein theposition determining apparatus comprises a transceiver capable ofreceiving signals from an RF transponder.
 18. The first mobile device asset forth in claim 17, wherein the first mobile device transmitsidentification information associated with the RF transponder to thecentral monitoring system.
 19. The first mobile device as set forth inclaim 14, wherein the at least one transceiver is capable oftransmitting a beacon signal to a plurality of base stations associatedwith the central monitoring system.
 20. The first mobile device as setforth in claim 19, wherein the central monitoring system uses beaconsignals received by the plurality of base stations to determine aposition of the first mobile device.